Diesel engine combustion results in the formation of nitrogen oxides, (NOx), in the exhaust gas. Typically, urea selective catalytic reduction systems (urea SCR systems) are used to reduce oxides of Nitrogen (NOx) from engines. Nitrogen oxides can be reduced by ammonia, however ammonia is difficult to store. For that reason, urea SCR systems rely on injection of 32.5% aqueous urea solution, which is injected into the exhaust line of a vehicle upstream of an SCR catalyst. In the SCR catalyst, the NOx is reduced by the ammonia, and the emission from the catalyst is N2, H2O and C02.
For efficient performance, the emission reductant, for example urea for SCR systems, must be injected into the engine exhaust gas, evaporized and decomposed before reaching the inlet of the aftertreatment catalyst. The residual time of the evaporation of the reductant depends on the physical properties of the reductant, the injection characteristics, and the energy of the exhaust gas. The heat required for evaporating most reductants is high. For example, a urea solution is injected into the system at an ambient temperature and typically needs to be heated above 150° C. or 200° C. to evaporate the water and decompose the remaining urea into ammonia and isocyanic acid.
When the urea or other reductant is sprayed into the system, the exhaust gas velocity is high, and the exhaust gas stream carries the large droplets at a high velocity to the catalyst. As a result, the residual time of the reductant evaporation is larger than the travel time to the catalyst. If the evaporation and the decomposition are not complete, the SCR catalyst performance is reduced due to insufficient availability of reductant. If the urea solution is not evaporated and decomposed before hitting the inner surface of the exhaust gas pipe, which is at a cooler temperature due to being exposed to the ambient, the urea solution will remain liquid and will not decompose. Further, the urea can form a solid deposit on the inner surface of the exhaust gas pipe.
To facilitate evaporation and decomposition of the reductant, a mixer has been installed in front of the SCR catalyst. Additionally, the reductant injector was placed as far away as possible upstream of the SCR catalyst. However, with larger engines having exhaust gas flow of 170 m3/hour to 1130 m3/hour, the residual time of the reductant evaporation is still larger than the travel time to the catalyst.